What are the diagnostic criteria and management strategies for Acute Respiratory Distress Syndrome (ARDS)?

ARDS Diagnosis

ARDS is diagnosed using the Berlin Definition, which requires all four criteria present simultaneously: acute onset within 1 week of a known clinical insult, bilateral opacities on chest imaging not fully explained by effusions/nodules/collapse, respiratory failure not fully explained by cardiac failure or fluid overload, and hypoxemia with PaO₂/FiO₂ ≤300 mmHg measured with minimum PEEP of 5 cmH₂O. 1

Diagnostic Criteria

Core Requirements

• Timing: Acute onset within 1 week of a known clinical insult or new/worsening respiratory symptoms 2
• Imaging: Bilateral opacities on chest radiograph or CT scan not fully explained by effusions, lobar/lung collapse, or nodules 2, 1
• Oxygenation: PaO₂/FiO₂ ≤300 mmHg with minimum PEEP of 5 cmH₂O 2, 3, 1
• Exclusion: Respiratory failure not fully explained by cardiac failure or fluid overload 2, 1

Severity Classification

ARDS severity is stratified based on the degree of hypoxemia, all measured with minimum PEEP of 5 cmH₂O 2, 3, 1:

• Mild ARDS: 200 < PaO₂/FiO₂ ≤ 300 mmHg 2, 3
• Moderate ARDS: 100 < PaO₂/FiO₂ ≤ 200 mmHg 2, 3
• Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg 2, 3

Mortality increases with severity, ranging from 35-40% overall to 46-60% in severe ARDS 2, 4

Essential Diagnostic Workup

Mandatory Initial Tests

• Arterial blood gas: Required to calculate PaO₂/FiO₂ ratio and assess for metabolic or respiratory acidosis 1
• Chest imaging: Radiograph or CT to document bilateral opacities 1
• Echocardiography: Perform when no clear ARDS risk factor exists to exclude cardiogenic pulmonary edema and assess for signs of fluid overload 1

Etiological Investigation

When no obvious cause is apparent or direct lung injury is suspected, bronchoalveolar lavage (BAL) should be performed to identify infectious organisms responsible for pneumonia. 5

The diagnostic workup should focus on identifying treatable causes 5:

• Blood cultures and biomarkers: Identify microorganisms and evaluate infection markers 5
• Second-line testing: If no infectious cause found, evaluate for immunologic diseases 5
• CT scan: Essential when suspecting intra-abdominal sepsis or complex pneumonia 5
• Lung ultrasound: Evaluate for bilateral diffuse areas of reduced lung aeration, interstitial syndrome, consolidations, pleural line abnormalities, and possible pleural effusion 1
• Open lung biopsy: Reserved for selected cases when all other examinations remain inconclusive 5

Critical Diagnostic Pitfalls

ARDS-Mimics Requiring Specific Treatment

Conditions like diffuse interstitial lung diseases, diffuse pulmonary infections, and drug-induced lung injury can present identically to ARDS and fall within the syndrome definition but require specific treatments rather than standard ARDS management. 2, 1

These ARDS-mimics may respond to 2:

• Immunosuppressants (corticosteroids) for interstitial lung diseases
• Specific antimicrobial drugs for identified pathogens
• Withdrawal of offending agents in drug-induced lung injury

Common Diagnostic Errors

• Calculating PaO₂/FiO₂ without adequate PEEP: Always use minimum 5 cmH₂O PEEP to avoid misclassification of severity 1
• Assuming pathology: Only a minority of patients meeting Berlin criteria actually have diffuse alveolar damage on autopsy, highlighting significant heterogeneity 1
• Missing cardiogenic causes: Actively exclude cardiogenic pulmonary edema through clinical assessment and echocardiography 1
• Ignoring underlying etiology: Adequate treatment of the responsible cause is crucial to improve outcomes 5

Management Strategies

Lung-Protective Ventilation (Foundation of Care)

Implement low tidal volume ventilation (4-8 ml/kg predicted body weight) with plateau pressure <30 cmH₂O in all ARDS patients. 2, 3

• PEEP selection: Use higher PEEP in moderate to severe ARDS, guided by gas exchange, hemodynamic status, lung recruitability, end-expiratory transpulmonary pressure, or driving pressure 2, 3
• Oxygenation target: PaO₂ 70-90 mmHg or SaO₂ 92-97% 2, 3
• Driving pressure: Monitor and minimize (plateau pressure minus PEEP) 3
• Consider esophageal pressure measurement: To guide PEEP selection 2, 3

Adjunctive Therapies for Moderate to Severe ARDS

For PaO₂/FiO₂ <150 mmHg, implement prone positioning for >12 hours daily and consider neuromuscular blocking agents. 2, 3

• Prone positioning: Implement for >12 hours daily in severe ARDS (PaO₂/FiO₂ <100 mmHg) 2, 3
• Neuromuscular blockade: Consider cisatracurium for 48 hours in early severe ARDS 2, 3
• Corticosteroids: Consider in selected patients, particularly when initiated early 3
• Recruitment maneuvers: Avoid prolonged recruitment maneuvers 3
• ECMO: Consider venovenous ECMO as rescue therapy for very severe ARDS failing conventional management 2, 3

Fluid Management

Implement conservative fluid management strategy once shock is resolved to avoid worsening pulmonary edema. 3

• Ensure adequate intravascular volume while avoiding fluid overload 3
• Monitor for iatrogenic injury from excess fluid administration, transfusions, or injurious mechanical ventilation 2

Noninvasive Support

Noninvasive ventilation with close monitoring is reasonable in less severely ill patients (mild ARDS), but high tidal volumes and strongly negative intrathoracic pressures can cause patient self-inflicted lung injury (P-SILI). 2

• Around 16% of ARDS patients may be managed with NIV or high-flow nasal oxygen 2
• Helmet interface may be most effective in acute respiratory failure 2
• Failure rates increase with moderate or severe hypoxemia (PaO₂/FiO₂ <200 mmHg) 2

Treatment of Underlying Cause

Adequate treatment of the underlying disease is fundamental to ARDS care and should be initiated as soon as the etiology is identified. 2

The rapid identification and treatment of specific causes (such as dexamethasone for severe COVID-19 pneumonia) demonstrates the importance of phenotype-aware management 2